The defending champion, Serena Williams, produced a remarkable comeback from a set and 4-0 down to beat Victoria Azarenka and reach the semi-finals of the Australian Open.

The world No1, initially bullied by the big-hitting seventh seed from Belarus, eventually came through 4-6, 7-6 (7-4), 6-2 and will meet China's Li Na for a place in the final. Li had earlier knocked out Serena's sister Venus on the Rod Laver Arena.

Serena appeared to be on the brink when Azarenka held serve to take a 4-0 lead in the second. But the four-time champion finally rediscovered her power game and serve, reeling off five straight games as she took the set on a tie-break. She then blew the 20-year-old off the court in the decider, seizing on both of her break-point chances.

"I'm surprised. I didn't expect to win when I was down 0-4," said Serena, who admitted she had even started thinking about the trip home. "I was actually thinking 'If I lose today and I lose in doubles I think I can catch a flight on Friday'. That's not what a champion is supposed to think, but that's what I was thinking. I'm just happy to still be here."

Asked about facing Li next, Serena said: "I have played her a few times and she has beaten me once [in Stuttgart in 2008]. I know her game well and the way I feel, I have nothing to lose."

Li rallied from a set and a break down, breaking Venus's serve to stay in the contest in the second set, on her way to a 2-6, 7-6 (7-4), 7-5 triumph. The 27-year-old - who had previously not been beyond the quarter-finals in a grand slam - proved slightly stronger in an edgy final set that included nine breaks of serve.

The 16th seed was broken to love when first serving for the match at 5-4, but she broke again and needed three match points before eventually sealing the win with a forehand winner down the line after two hours and 47 minutes.

Li's progress to the semi-finals means China have two players in the last four of a grand slam for the first time, with Zheng Jie already through to face Justine Henin in the other semi-final. "It's the best day of my whole life," said Li, grinning widely. "Good for both players. Also good for China's tennis. Also good for the fans."

It was another disappointing exit for the 29-year-old Venus, who has never won the Australian Open and has failed to progress past the last eight at Melbourne Park since 2003. "In tennis you have to close it out," said Venus. "It's not like there's a clock ticking and then suddenly it's over. You just have to close it out. I didn't do that today."

Venus then bristled at the suggestion that her only chance at another major was at Wimbledon, where she last won in 2008. "Like I said so many times before, if I would have gotten involved in what people said, I would have never left the ghetto. So here I am playing pro tennis, playing well. The sky's the limit in this sport. I'm looking for that."